Eyeless hydra sheds light on evolution of the eye

One of creationists' favourite claims is that an organ as intricate as the eye could never have simply evolved. Fresh evidence to the contrary has now arrived, courtesy of a creature related to jellyfish.

The tiny freshwater hydra has no eyes but it will contract into a ball when exposed to sudden bright light. David Plachetzki and colleagues at the University of California, Santa Barbara, have found that hydras "see" light using two proteins closely related to those in our own eyes.

"If you look at something as complex as an eye, you might be at a loss to explain how the whole structure evolved at once," says Plachetzki, now at the University of California, Davis. "But if you look at its components you can start to piece together how it happened." That's especially feasible now that genes from the earliest animals, such as the hydra, are being sequenced.

Rod and cone cells in the human retina contain proteins called opsins that change shape when light strikes them. This causes another type of protein, an ion channel, to generate an electrical signal along nerves connecting the eye to the brain – a process called phototransduction.

Hydras have the same types of opsins and ion channels as we do. Plachetzki's team found that they make them together in nerve cells. Moreover, they found that a drug that blocks those channels stopped hydras responding to light, showing they are used for phototransduction.

Seeing like us

"This is conclusive evidence that these animals, the Cnidaria, have light sensitivity based on this kind of opsin and transduction, just as we do," says Dan Nilsson of the University of Lund in Sweden, who has also investigated hydras' light reception.

Plachetzki's team then built a family tree of opsin gene sequences from 22 highly diverse creatures, and found that opsins in hydras and humans evolved from those in a common ancestor. Another line of descendants from the same ancestor gave rise to somewhat different opsins and ion channels in insect and mollusc eyes. This supports other indirect evidence, says Nilsson, that the hydras' light-sensing equipment was the original model, and the insects' came later.

The hydra is the most primitive animal with functioning opsins, so the team concludes that it represents "the very origin of animal phototransduction", which was incorporated into more complex eyes as they evolved.

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.